6-MP is an effective antileukemic agent, however little is known regarding its ability to cross the blood-brain barrier. Our findings indicate that the drug levels from the CSF and plasma decline in parallel with the elimination half-life of approximately 2.4 hours. Plasma/CSF ration in the past distributive phase ranged between 1-3 suggesting that significant concentration of 6-MP can be achieved in the CSF following IV bolus and may give clinical significance in the treatment of patients with acute lymphoblastic leukemia.